1. Field of the Invention
The present invention relates to a rubber member for use in a developing roller, a cleaning roller, a cleaning blade, a charging roller, and the like to be mounted on an electrophotographic apparatus. More particularly, the present invention relates to a rubber member for use in a developing roller to be mounted on an image-forming mechanism of the electrophotographic apparatus in which an unmagnetic one-component toner is used to transport the toner to an electrophotographic photoreceptor by imparting a high electrostatic property thereto.
2. Description of the Related Art
In recent years, in the printing technique using an electrophotographic method, a high-speed printing operation, formation of a high-quality image, formation of a color image, and miniaturization of image-forming apparatuses have been progressively made and become widespread. Toner holds the key to these improvements. To satisfy the above-described demands, it is necessary to form finely divided toner particles, make the diameters of the toner particles uniform, and make the toner particles spherical. Regarding the technique of forming the finely divided toner particles, toner having a diameter not more than 10 μm and not more than 5 μm have been developed recently. Regarding the technique of making the toner spherical, toner having not less than 99% in its sphericity has been developed.
To form the high-quality image, polymerized toner has come to be widely used instead of pulverized toner conventionally used. The polymerized toner allows the reproducibility of dots to be excellent in obtaining digital information as a printed sheet and hence a high-quality printed sheet to be obtained. It is possible to adjust the degree of the electrostatic property of the polymerized toner more easily than the pulverized toner. Further it is possible to prevent a variation of the particle diameters of the polymerized toner to be filled in a cartridge and a variation of degrees of the electrostatic property thereof.
In recent years, development of compact, lightweight, and inexpensive image-forming apparatuses are demanded owing to spread of personal use of the image-forming apparatus represented by printers and owing to a demand for space-saving of an office. On such a background, instead of a two-component toner containing magnetic powder which is capable of realizing the formation of a high-quality image but is an obstacle in miniaturizing the image-forming apparatus and making it lightweight, the use of a one-component toner not using the magnetic powder is rapidly spreading.
When the two-component toner using the magnetic powder is used, toner can be transported to the electrophotographic photoreceptor comparatively easily owing to electric and magnetic actions. But when the unmagnetic one-component toner is used, it is impossible to utilize the magnetic action in transporting the toner. Therefore it is necessary to uniformly form the surface of the developing roller which is an electrode end surface. To uniformly attach toner having small diameters of micron order to the surface of the developing roller, the electrical properties of the developing roller represented by the electric resistance value are demanded to be very uniform inside the developing roller so that when a bias electric potential is applied to the developing roller, a very uniform electric potential distribution is obtained.
Because the one-component toner does not contain magnetic toner, the developing roller is demanded to have a function of controlling the degree of the electrostatic property of the toner. That is, the developing roller is demanded to charge the toner and keep the electrostatic property imparted to the toner. If the toner has an insufficient charged amount, it has an insufficient electrostatic force. Thereby the toner is not faithfully transported to an electrostatic latent image formed on the electrophotographic photoreceptor. Thereby various defective images are generated. For example, there occurs a variation in the print density owing to a rotation of the developing roller, a development ghost, a photographic fog, and the like.
To comply with the above-described demands, a developing roller having the base material consisting of silicone rubber and the surface layer, consisting of urethane coating, which is disposed on the base material has been developed and used recently. But the silicone rubber used as the base material of the developing roller is expensive, and the yield is low in the step of forming the urethane coating. Such being the case, researches are now made to develop a developing roller, composed of ionic-conductive vulcanized rubber, which can be produced at a low cost and easily controlled in the electric resistance value thereof.
For example, in the conductive rubber roller disclosed in Japanese Patent Application Laid-Open No. 2004-170845, the outermost layer is composed of the ionic-conductive rubber to which a specific dielectric loss tangent-adjusting filler is added to adjust the dielectric loss tangent thereof to the range from 0.1 to 1.5.
The above-described conductive rubber roller provides a very high-quality image in various environmental conditions. In the case of a durability test, it is possible to prevent photographic fog from occurring because the charged amount of toner does not decrease and prevent toner from leaking mainly from a sealing portion of the roller. Normally, toner leak occurs owing to wear of the roller. Thus the conductive rubber roller can be used as a preferable developing roller.
When the above-described developing roller is used at a low temperature and a low humidity at an earlier time of the life of a toner cartridge when toner has been appropriately used and is apt to be charged, the electric resistance value of the roller rises because the outermost layer thereof is composed of the ionic-conductive rubber. Thereby the charged amount of the toner increases. Consequently the print density is liable to drop. Thus the roller has room to be improved in this respect.
In Japanese Patent Application Laid-Open No. 2005-225969 (patent document 2), there is disclosed the rubber member in which wax is added to the ionic-conductive rubber component. According to the disclosure made in the example of the specification, when the rubber member is used as a developing roller, a favorable initial image is formed. This is because the surface free energy decreases owing to the addition of the wax to the ionic-conductive rubber component, and toner separates from the roller favorably. As a result, there is an increase in the print density.
But the rubber member has room to be improved in the print density in the low temperature and humidity condition.
Patent document 1: Japanese Patent Application Laid-Open No. 2004-170845
Patent document 2: Japanese Patent Application Laid-Open No. 2005-225969